1. Field of the Invention
This invention relates to a method for the dry slaking of calcium and magnesium oxides from calco-magnesian compounds preferably containing at least 10% by weight of MgO relative to the total weight of said calco-magnesian compound, comprising the steps of:
feeding calco-magnesian compound containing MgO into slaking equipment;
feeding an aqueous slaking phase into said slaking equipment; and
slaking the said calcium and magnesium oxides of the said calco-magnesian compound fed into the said slaking equipment, with said aqueous slaking phase, leading to the formation of slaked solid particles of calcium and magnesium hydroxides (e.g. of general formula xCa(OH)2.yMg(OH)2).
2. Description of the Prior Art
Typically, the slaking of MgO to form magnesium hydroxide Mg(OH)2, compared with conventional slaking of CaO to form Ca(OH)2, is a difficult reaction to carry out profitably at industrial level. One of the possible routes to produce fully hydrated products, namely in which the MgO and CaO are fully converted to Mg(OH)2 and Ca(OH)2 respectively, is high pressure hydration during which a supply of heat is sometimes necessary as a function of the reactivity of the MgO. In some cases during wet process slaking, very long reaction times may overcome these drawbacks. However, the use of high pressure hydrators with or without the supply of heat, or long residence times are as many obstacles which reduce the industrial production of magnesium hydroxide from MgO, preference being given to the chemical production of Mg(OH)2 from salts treated with caustic solutions.
Just as there exist limestone deposits (calcium carbonate of formula CaCO3), there also exist dolomite deposits (mixed calcium and magnesium carbonate of general formula xCaCO3.yMgCO3 where x and y are molar coefficients such that x/y=0.8 to 1.2), and deposits of other mixed carbonates of general formula xCaCO3.yMgCO3 where 1.2≦x/y≦25) or of any other mixed carbonate of calcium and magnesium. In the same manner as for limestones, dolomites or the other aforementioned mixed carbonates can be calcined thereby producing dolomitic quicklime of formula xCaO.yMgO or any other mixed calcium and magnesium oxide. Similar to quicklime, a hydration reaction is initiated when dolomitic quicklime or any other aforementioned mixed oxide is contacted with water. However, as mentioned above, it is known that magnesium oxide has much lower reactivity to water than high calcium quicklime that is highly hydrophilic, the hydration of these calcined compounds thereby becoming much more complex than the hydration of high calcium quicklime.
For this reason, in a large majority of cases, a hydrated dolime, or other mixed calcium and magnesium hydroxide produced industrially by dry process is in fact a semi-hydrated dolime or other aforementioned mixed hydroxide containing a non-negligible amount of residual non-hydrated MgO. The above-mentioned semi hydrated dolime is generally represented by the formulas xCa(OH)2.yMgO or xCa(OH)2.(y-z)Mg(OH)2.zMgO depending on the hydration level of the magnesium oxide (where x/y=0.8 to 1.2). The other aforementioned mixed hydroxides have formulas of same type but with different calcium/magnesium molar ratios.
To overcome this low reactivity to water of quick dolime or other aforementioned calcined compound, with a view to obtaining fully hydrated products (of general formula xCa(OH)2.yMg(OH)2) it is known to have recourse to a much greater amount of water than the amount of water used for dry slaking and/or higher temperatures than those prevailing in standard hydration reactions for quicklime, or milling of the dolime or other aforementioned calcined compound prior to hydration, even to a hydrothermal reaction i.e. hydration under pressure at high temperature (under water vapour, 7-13 bars, 115-165° C.) (Boynton, Chemistry and Technology of Lime & Limestone, 2nd edition, Wiley Interscience, pp. 325-326 and pp 374). A dolomitic product of this type that is fully hydrated is known as Type S as opposed to Type N which represents normal dolomitic hydrates i.e. partly hydrated (OATES, J.A.H. Lime and Limestone, Weinheim: Wiley-VCH, 1998, pp. 222-223).
In all cases these slaking processes are complex and costly and reduce the industrial use of these MgO-containing compounds.
FR2669327 A1 teaches the use of chemical agents such as amines or glycols to produce a hydrated lime at regulated or controlled temperature having improved plastic properties and high specific surface areas.
WO9209528 also concerns the use of additives selected from among glycols and/or amines to produce calcium and/or magnesium hydroxides having high specific surface areas.
U.S. Pat. No. 3,120,444 discloses the addition of low molecular weight aliphatic alcohols to obtain hydrated lime with improved plastic properties.
U.S. Pat. No. 1,856,763 discloses a method to produce quicklime of high purity with delayed reactivity for use thereof in mortars.